You must configure OpenSSH Authentication Agent service to automatically start (or you can start it manually everytime when opening your powershell for the first time: Start-Service ssh-agent).

After that, you need to ssh-add C:\path\to\your\ssh\key\id_rsa only once. After that, everytime the ssh-agent is started, the key will be there. You can check with ssh-add -l.

EDIT: To have SSH agent to automatically start with Windows, you can run Set-Service ssh-agent -StartupType Automatic on a super-user powershell prompt.